Chapter 29
Observational Calculations
vector: d1 =
Horizontal distance at the elevation of the source point
h1 =
Elevation of the source point
ht
=
Elevation of the target point
R
=
Radius of the spheroid
( h 1 + h t )d 1 d 2 = d 1 – --------------------------2R
Reduction of the sea level chord (d2) to the spheroidal arc (d3) involves a correction of the following: d 23 -----------24R 2 R
=
Radius of the spheroid
This correction exceeds 1mm only on distances greater than 9.9km. Consequently, the correction term is ignored and the spheroidal arc is taken to be the sea level chord: d3 = d2
29.3.3
Projection correction The correction of the spheroidal arc (d3) to a projected distance (d4) depends on the projection used. Because the locally used projection is not known by the SRX, a simple scale factor is used. The value of the scale factor is specified during job creation. For short and medium distance EDM work, this provides sufficient accuracy. The projection correction is as follows: sf
=
Scale factor in the current scale record
d 4 = d 3 + sf
29.3.4
Slope reduction Refer to Figure 38 to better understand this reduction. The horizontal and vertical components (d1 and v) of an observation are found from the vertical angle (zenith distance) and slope distances by the following: d1
=
Ssin(Z)
v1
=
Scos(Z)
Z
=
Zenith angle
S
=
Slope distance
29.4 Other Formulas The two formulas covered in this section include:
29-6
SRX